Refusal to accept a credit card for purchases under R50

I had just purchased something for R70 with my credit card, and on the way out spotted a few smaller items which totaled R43. The proprietor refused to accept my credit card for this, citing their 'rule' that no purchases under R50 can be made with a credit card. He wouldn't offer a justification for this 'rule', and wasn't interested to hear that he is not in fact allowed to do that. So, I left without the additional goods, and he lost the sale. I'll be reporting the merchant to ABSA bank, whose credit card machine they are using.